The present invention relates generally to methods and apparatus for desolventizing solids; and more particularly to a process for removing residual solvent from a solid such as soybean meal.
Organic solvents such as hexane, heptane, alcohols and the like are commonly used for treating or processing solids, to extract a useful material from the solid and/or to cause a desired change in that solid. For example, organic solvents (e.g., hexane) are used to extract oil and fat from oilseed such as soybean. The typical soybean oil-extraction process has many steps, including cleaning, cracking, conditioning, expelling and solvent extraction, resulting in a solid (i.e., oil-depleted meal) and a liquid (i.e., oil). The oil can then be further refined and processed to make such things as margarine and shortening; various deodorized products; and various physically refined products. The oilseed meal, on the other hand, can be used for such things as animal feed. But in order to do so, it must have its residual solvent removed. After extracting oil and/or fat from the soybeans a quantity of the hexane remains entrained or entrapped within the solid particles, and in order to make maximum use of the oil-depleted solid particles it is important to remove as much of the residual solvent as possible. Before the deoiled (or defatted) spent meal can be used as cattle or chicken feed it must first have the residual n-hexane (typical) removed.
The traditional method for removing the solvent from the spent soy is to heat the solids under a slight vacuum and allow the solvent to boil from the solids. This is called desolventizing or desolventizing-toasting (DT). While these traditional methods for removing the residual solvent from the oilseed meal are sometimes satisfactory, it is often desirable to remove even more of the residual solvent.
A DT may be used to remove a good portion of the solvent, but often residual solvent remains. Inside the extractor the materials undergo a separation process where the solids are separated from the liquids. And then the solids may be processed by a DT. But in either case, it is common for there to be some solvent remaining entrapped or entrained by or in the solid particles. The present invention relates to a process for removing residual solvent from a solid such as an oilseed meal that has advantages over the traditional methods in many cases. For example, the present process can strip more residual solvent than traditional methods, and do so in such a way that is often more energy efficient and less damaging to the solid.
The present invention is a process for removing residual amounts of a first solvent from a granular material. The process applies to solvent removal wherein the first solvent has a first ambient pressure boiling point. The process includes a first step of selecting a second solvent, having a second ambient pressure boiling point, that is lower than the first ambient pressure boiling point and mixing the second solvent with the granular material and first solvent. Most of the liquid solvent mixture is then removed from the granular material using a simple solid/liquid extraction process. Finally, the remaining solvent mixture, which is primarily made up of the second solvent, is removed from the granular material using a thermal and/or vacuum process. This serves to leave a highly desolventized granular material.
In a preferred manner of performing the process, the liquid solvent mixture removed from the granular material by use of a simple solid/liquid extraction process is then distilled in order to recover liquid original solvent and a gaseous second solvent. Such recovered gaseous products can then be compressed and chilled to form a liquid second solvent. If desired, such a liquid product can be returned to the mixer.
The process in accordance with the present invention is particularly efficacious for removing solvent from organic materials such as seeds, berries and leaf particles. It also serves well to remove solvent from inorganic or inert solids such as silica sand. Further, the solids may be granular or thin plates or a woven material forming a web or a combination thereof.
The present invention is thus an improved process for solvent removal from granular materials. More specific features of the invention and advantages obtained in view of those features will become apparent with reference to the DETAILED DESCRIPTION OF THE INVENTION, appended claims, and accompanying drawing.